Sunburn has been a problem for apple growers for at least 75 years, but its incidence has increased in recent years with the widespread use of dwarfing rootstocks and high-density plantings. Many cultivars (e.g., ‘Fuji,’ ‘Granny Smith,’ ‘Jonagold,’ ‘Gala,’ and ‘Braeburn’) are susceptible to sunburn. Prominent growers have indicated that sunburn may be the most significant cullage or quality problem in the industry. Trees are smaller and fruit are more exposed to solar radiation making fruit more susceptible to sunburn.
There is no adequate product on the market today for preventing sunburn damage. Many growers use overhead evaporative cooling or shadecloth to reduce sunburn in their apple orchards. Evaporative cooling decreases the temperature of the fruit and helps protect the fruit from sunburn (Parchomchuk, P. and Meheriuk, M., “Orchard cooling With Pulsed Overtree Irrigation to Prevent Solar Injury and Improve Fruit Quality of ‘Jonagold’ Apples,” HortScience 31:802-804 (1996)). However, growers are concerned about several deleterious effects on fruit trees and soil (Warner, G., “Overhead Cooling May Not Be Total Sunburn Cure,” Good Fruit Grower 46(12):20-21 (1995)). The shadecloths cost several thousand dollars per acre to install, and frequently interfere with normal color development of fruit. Uniform shade causes an undesirable alteration in the growth habit of apple trees and significantly reduces fruit production (Warner, G., “Cooling Problems Prompt Growers To Try Covers,” Good Fruit Grower 46(12):24-25 (1995); Warner, G., “Growers Test Shade Cloths To Reduce Fuji Sunburn,” Good Fruit Grower 46(17):55-63 (1995); Warner, G., “What Shade Do Cloths Provide, What Do You Need?”, Good Fruit Grower 46(17):50-53 (1995)). Problems with these approaches confirm that new treatments are needed to lower fruit temperature, but not interfere with color development or fruit production.
In 1986 and 1987, Sibbett et al. (“Effect Of A Topically Applied Whitener On Sun Damage To Granny Smith Apples,” California Agriculture 45(1):9-10 (1991)) in California attempted to solve the problem by applying a commercial whitener (Sunguard) to Granny Smith apples. The whitener had been developed for walnuts. They concluded from their experiments that Granny Smith apples could not be protected from sunburn by up to four topical applications of this particular whitening agent.
Miller Chemical & Fertilizer Corp. (Hanover, Pa.) markets an anti-transpirant concentrate called VAPOR GARD, and claims in its advertisements that the product reduced sunburn cullage by 30% in their trials. Transpiration is important to plant leaves, as evapotranspiration serves to cool the leaves and protects the leaves from heating to temperatures that are deleterious. Fruits have much lower transpiration rates than do leaves, but it seems likely that applying an anti-transpirant to fruit would exacerbate a situation in which there is already too much thermal energy.
Myhob, Guindy, and Salem in Egypt (Bulletin of Faculty of Agriculture, University of Cairo, 47(3):457-469 (1996)) reported that Agricultural GatCool significantly reduced sunburn as compared to controls sprayed with water on Balady mandarin fruits. duToit in South Africa (Citrus and Subtropical Fruit Research Institute Information Bulletin No. 80:8-9 (1979)) reported that spraying Koolcote on pineapple trees decreased fruit flesh temperatures by 2-3 degrees Celsius.
Lipton and Matoba (HortScience 6(4):343-345 (1971)) reduced sunburn of ‘Crenshaw’ melons by whitewashing fruit with a suspension of aluminum silicate.
Ing (Good Fruit Grower 49(6):58 (1998)), commenting on unpublished field trials, reports that the application of kaolin to apple fruits not only acts as an insect repellent, but also lowers canopy temperature, increases fruit size, and may reduce sunburn. However, as noted by Ing, application of kaolin to fruit surfaces is problematic. To achieve an insecticidal result, large amounts of kaolin (50 to 100 pounds per acre) must be applied to the fruit trees. Current kaolin formulations are reported to suffer from substantial application problems such as excessive foaming and “globbing” in spray tanks. (Good Fruit Grower 49(6):58 (1998)). Furthermore, kaolin powders are easily washed off by rain, thus necessitating multiple applications in order to maintain beneficial effects. (Good Fruit Grower 49(6):58 (1998); see also Washington State University Cooperative Extension Area Wide IPM Update 3(4):1 (1998)).
Sekutowski et al. (U.S. Pat. No. 5,908,708) developed a protective water resistant coating that was formulated as an aqueous dispersion of particulate matter having a hydrophobic outer surface in a low boiling point organic liquid, such as methanol. The particulate matter of the Sekutowski et al. coating can be any finely divided hydrophobic particulate solids including minerals, such as calcium carbonate, mica, talc, kaolin, bentonites, clays attapulgite, pyrophyllite, wollastonite, silica, feldspar, sand, quartz, chalk, limestone, precipitated calcium carbonate, diatomaceous earth and barytes. One agricultural use of the Sekutowski et al. aqueous dispersions is to provide tree leaves with a water resistant coating by spraying the formulation onto the surface of the leaves. The water resistant coating is thought to reduce plant disease and insect damage. However, one major problem with the Sekutowski et al. formulation is the use of large volumes of organic liquids such as alcohols, ketones and cyclic ethers that are highly flammable and pose other health risks to workers during spray application.
Protective formulations which additionally function as pesticides in plant crops would be a valuable addition to Integrated Pest Management (IPM) practices providing “soft” suppression of pests without disrupting natural control processes. Desirable formulations would be expected to be non-toxic to mammals and thus safe for applicators and farm workers. Application of the protective formulations by commonly employed horticultural spray operations invariably involves treatment of foliage and fruit or vegetable. It is therefore important to develop new formulations that have protective properties against sunburn to fruits and vegetables as well as against damage caused by insects that inhabit both foliage and fruit.
In summary, there is a lack of adequate means to prevent sunburn and insect damage to fruit and vegetable crops. Thus, there is a strong need in agricultural markets for an inexpensive and effective composition that prevents sunburn, repels deleterious insects, is long lasting, and is relatively amenable to easy application by growers and commercial applicators.